Air separation is widely practiced and one well known process generally involves the cryogenic distillation of air in a distillation system comprising a main distillation zone for separating nitrogen and oxygen and an argon side-arm column wherein crude argon is recovered from the cryogenic distillation process. Typically, the distillation system comprises a double-distillation column wherein air is introduced to a high pressure column and a low pressure column. An argon side stream is generated in the low pressure column and withdrawn from that column for further refinement. Any nitrogen in the argon stream is recovered as an overhead from the argon side column along with the argon. Typically, the overhead stream will contain from 2-5 mol% oxygen and 1 mol% of nitrogen. The balance of the stream comprises argon. Oxygen is withdrawn from the bottom of the column.
Argon recovery and purification for use in metallurgical and electronics applications is important to the air separation industry. There have generally been two approaches used for the further refinement of a crude argon stream to produce high purity argon. One technique has been referred to as catalytic hydrogenation and is effected by contacting the crude argon stream with a hydrogen containing atmosphere in the presence of a metal such as nickel, palladium, or a metal getter whereby the residual oxygen is reacted with the hydrogen generating water vapor. This stream then is cooled and cryogenically distilled for removing the nitrogen therefrom. Alternatively, the crude argon stream may be purified by a treatment process referred to as cryogenic adsorption. In that technique nitrogen is initially removed by contacting with an adsorbent suited for the preferential adsorption of nitrogen and then the essentially nitrogen free argon stream is contacted with an adsorbent suited for the preferential adsorption of residual oxygen in the stream. Representative patents which disclose variations of the catalytic hydrogenation process are as follows:
U.S. Pat. No. 4,994,098 discloses a cryogenic process for the preparation of crude argon. The cryogenic process typically involves a three column system wherein there is a high pressure, low pressure and an argon column in communication with each other. A structured or ordered packing is used in at least a portion of the argon column to promote liquid and vapor mixing with minimal pressure drop in the argon column. As a result, greater separation of argon from oxygen is achieved and a crude argon stream having reduced oxygen content is generated. The oxygen concentration typically is less than 0.5 mol%.
U.S. Pat. No. 4,983,194 discloses a double-distillation column system for the separation of air incorporating an argon side-arm column. Crude argon stream having an argon purity of less than about 0.8 mole % oxygen is withdrawn from the column, condensed and then subsequently vaporized prior to passing the stream over a bed of one or more reduced metal getters on a suitable catalyst support. Representative metal getters include copper, nickel or combinations thereof which are regenerable by reduction with hydrogen.
Representative patents showing the removal of oxygen or nitrogen or both from a crude argon stream using cryogenic adsorption techniques are shown in the following patents:
U.S. Pat. No. 3,928,004 discloses passing a crude argon stream having less than about 3.5% nitrogen and more than 1% oxygen through a molecular sieve bed which preferentially adsorbs oxygen. This molecular sieve typically is a 4A or sodium exchanged molecular sieve and cryogenic adsorption is effected at temperatures of about -275.degree. F. (-170.degree. C.) for effecting removal of the oxygen. The oxygen is desorbed from the sieve by evacuation of the bed and flushing with an inert gas.
U.S. Pat. No. 3,996,028 discloses a process for the purification of a crude argon stream containing oxygen by passing the argon stream through synthetic zeolites of the A type having entry voids from 2.8 to 4.2 Angstroms. The oxygen is adsorbed at a pressure of 21.38 to 427 psia and desorbed by reducing the pressure of atmospheric pressure with subsequent vacuum treatment of the zeolites typically, 1-10.sup.-2 mm Hg. By using cryogenic adsorption, the patentees where able to overcome disadvantages associated with catalytic hydrogenation using a hydrogen as had been used in the prior art. The patentees overcame problems associated with the cryogenic adsorption of oxygen from argon by using a refrigerant comprising liquified nitrogen, liquefied oxygen, and mixtures thereof or liquefied argon boiling under gauge pressure. Utilization of these refrigerants eliminated the formation of bimers of argon and oxygen and it eliminated a number of disadvantages associated with the use of liquid oxygen as a refrigerant. In addition the use of the mixture of gases afforded an opportunity for easy change of adsorption temperature by adjustment of the refrigerant pressure.
U.S. Pat. No. 4,239,509 discloses a process for the cryogenic adsorption of oxygen and nitrogen from a crude argon stream which offers advantages over the processes described in the '004 and '028 patents just described. The process involves purifying a crude argon stream containing approximately 2% oxygen and 0.5% nitrogen by passing the crude stream through a molecular sieve suited for the preferential adsorption of nitrogen, e.g., a 5A molecular sieve at a temperature of about -280.degree. F. such that the stream exiting the 5A molecular sieve does not exceed a temperature of about -250.degree. F. That stream then is passed through a bed containing a molecular sieve suited for the preferential adsorption of oxygen, e.g., a 4A molecular sieve. Residual oxygen is removed from the stream. To maintain a bed temperature of a least -250.degree. F. or below during the removal of oxygen, the molecular sieve adsorption system was designed such that the 5A molecular sieve adsorption system encapsulated the 4A adsorption system. By carrying the adsorption of nitrogen from the argon at a temperature well below about -250.degree. F., it was possible to maintain the 4A zone at a temperature of -250.degree. F. or below. Encapsulation of the adsorption zone containing the 4A molecular sieve with the adsorption zone containing the 5A molecular sieve eliminated many of the problems associated with the use of liquid oxygen and other refrigerants for maintaining bed temperature which were used in the prior art.